1. Field
The present disclosure is directed to a method and apparatus for beamforming with coupled antennas.
2. Introduction
Presently, wireless communication devices communicate with other communication devices using wireless signals. Many wireless communication devices have multiple antennas that can transmit more focused signals to a receiving device using antenna beamforming. For a transmitter with M antennas transmitting to a receiver with a single antenna, an optimal beamformer maximizes the signal-to-interference ratio at the receiver subject to a constraint on transmit power. The solution for maximizing the signal-to-interference ratio is known as the eigenbeamformer due to the fact that the optimal transmit weights correspond to the eigenvector of a matrix having the largest eigenvalue. While this method is known, the impact of antenna coupling at the transmitter has typically not been considered in the formulation. However, the effect of the antenna correlation must be considered in the formulation of the eigenbeamformer as it affects the computation of the radiated power, and thus the constraint on radiated power.
The effect of mutual coupling has been considered to some extent. U.S. Pat. No. 7,457,590, (issued Nov. 25, 2008), which is hereby incorporated by reference, addresses how to compute the eigenbeamformer as a function of the channel received angular power profile ƒ(θ) in the case that the antenna pattern pk(θ) resulting from each antenna weighting of the form
      u    k    =                    [                                            0              ⁢              …0                                      ︸                              k                -                1                                              ⁢          10          ⁢          …0                ]            T        .  
In “Mutual Coupling in MIMO Wireless Systems: A Rigorous Network Theory Analysis” (Wallace, IEEE, p. 1317-1325, Jul. 4, 2004), which is hereby incorporated by reference, the eigenbeamformer is not explicitly calculated. However, the impact of mutual coupling on the calculation of the radiated power is addressed in computing the MIMO channel capacity in the case of mutual coupling at the transmitter, though only for the case in which the antennas are driven by ideal current sources. Unfortunately, the technique does not address coupled antennas driven by actual sources.
Thus, there is a need for a method and apparatus for improved beamforming with coupled antennas.